Department of Earth and Environmental Sciences, Columbia University, New York, NY, 10025, USA.
Geosyntec Consultants, Huntington Beach, CA, 92648, USA.
Nat Commun. 2020 May 7;11(1):2244. doi: 10.1038/s41467-020-16104-z.
Confining clay layers typically protect groundwater aquifers against downward intrusion of contaminants. In the context of groundwater arsenic in Bangladesh, we challenge this notion here by showing that organic carbon drawn from a clay layer into a low-arsenic pre-Holocene (>12 kyr-old) aquifer promotes the reductive dissolution of iron oxides and the release of arsenic. The finding explains a steady rise in arsenic concentrations in a pre-Holocene aquifer below such a clay layer and the repeated failure of a structurally sound community well. Tritium measurements indicate that groundwater from the affected depth interval (40-50 m) was recharged >60 years ago. Deeper (55-65 m) groundwater in the same pre-Holocene aquifer was recharged only 10-50 years ago but is still low in arsenic. Proximity to a confining clay layer that expels organic carbon as an indirect response to groundwater pumping, rather than directly accelerated recharge, caused arsenic contamination of this pre-Holocene aquifer.
隔水粘土层通常可防止地下水中的污染物向下渗透。然而，在孟加拉国地下水砷的背景下，我们通过研究表明，从粘土层中提取的有机碳进入低砷全新世（> 12000 年前）含水层，会促进氧化铁的还原溶解和砷的释放。这一发现解释了在这种粘土层下方的全新世含水层中砷浓度的持续上升，以及结构完好的社区水井的反复失效。氚测量表明，受影响深度区间（40-50 米）的地下水是在 60 多年前补给的。同一全新世含水层中更深的（55-65 米）地下水是在 10-50 年前补给的，但砷含量仍然很低。与排出有机碳的隔水粘土层相邻，这是地下水抽取的间接反应，而不是直接加速补给，导致了该全新世含水层的砷污染。
